The Osprey as a Sentinel Species for Environmental Contaminants

One of my favorite birds is the Osprey (Pandion haliaetus), a large, fish-eating Falconiforme, and I’ve spent many happy hours watching these raptors, especially in the Pacific Northwest. I’ll admit I was drawn to the two papers by Henny and colleagues (2003; 2007) in large part because of the location of their research areas: the Columbia and Willamette Rivers, my old UO grad school stomping, hiking, and birding grounds. The more recent paper provides evidence that the concentrations of some environmental contaminants may be decreasing, and that Osprey populations in the research sites are increasing.

In the 2003 paper, Henny and colleagues report on application and development of several research methods used for monitoring biomagnification factors (BMF) for organochlorine pesticides and other contaminants, in Ospreys along the Willamette River, in Oregon. The BMF is a comparison of the levels of contaminants in Osprey eggs, with the levels of contaminants present in the major fish species consumed by the birds. The Osprey is an ideal species in which to study BMF for a number of reasons, including its long lifespan with high nest fidelity, its 99% fish diet, the exposed and accessible nest sites (often located on man-made structures), and its tolerance for short-term nest disturbance, with minimal impact on the local population by sampling removal of an egg from small subsets of nests.

Although some of the contaminants present in the Ospreys could have originated from their wintering grounds in Mexico and Central America, most were likely to have been obtained from fish in the Oregon breeding grounds. The species of fish that comprised the majority of the Osprey diet were determined from analysis of prey remains, and then composite samples of these species (including one omnivore = Largescale Sucker, one piscivore = Northern Pikeminnow, and one insectivore = Mountain Whitefish) were collected using boat electrofishing. Levels of organochlorine pesticides, polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) in both Osprey eggs and composite fish samples were measured using gas chromatography/mass spectrometry; fish samples only were analyzed for total mercury. The authors considered numerous variables and confounding factors in their discussion, including the size and migratory patterns of the prey fish species, alternative foraging sites and prey species for the Ospreys, and sources of industrial contaminants. Ospreys were found to have higher BMFs for DDE/DDT than did Herring Gulls and Brown Pelicans in earlier studies, and Ospreys are more sensitive to the shell-thinning effects of these pesticides. Only the DDE, possibly a residual lipophilic contaminant from wintering grounds, was believed to adversely affect the Willamette Osprey population, which has increased in numbers steadily since the 1972 DDT ban.

In a more recent paper, Henny and colleagues (2007) used many of the same methods to examine reproductive success and contaminant levels in Osprey populations along the Lower Columbia River, which separates the states of Oregon and Washington. First, the authors divided their study region into Reaches, taking into consideration the contaminants from agriculture, aluminum smelters, and bleached-kraft paper mills along the Columbia River drainage area. The population analysis component of the study indicated that between 1998 and 2004, the number of occupied Osprey nests increased from 103 to 225, with an annual increase rate of 13.9%. However, productivity, in terms of number of fledged young per nest, varied between the four Reaches, as did the amount of increase in this outcome between 1998 and 2004. Levels of all OC pesticides in Osprey eggs decreased between 1998 and 2004, with the levels of DDE well below those determined to cause detrimental eggshell thinning in this species. PCBs, PCDDs, and PCDFs all decreased significantly from 1998 levels. In fact, mercury was the only environmental contaminant that increased between 1998 and 2004.

The latter paper reflects a significant improvement in the status of breeding Osprey populations along the Lower Columbia River, especially considering that in 1997/1998 these birds had the highest DDE concentrations reported in North America. Ospreys seem to prefer manmade structures as nesting sites, such that few nest in trees, and the population density is lower above the Bonneville Dam, where fewer channel markers and similar structures are present. The combined effects of PCBs, PCDDs, and PCDFs on reproductive success among the Columbia River Ospreys were concluded to be negligible in the 2004 period; however, the increase in mercury levels warrants continued monitoring. Levels of polybrominated diphenyl ethers (PDBEs = flame retardants), not reported in this study, are also increasing in nearby areas, and should be monitored in Ospreys.

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[…] Guadalupe Storm-Petrel wrote an analysis of several peer-reviewed papers about Osprey as sentinals for environmental contamination and finds that the concentrations of some environmental contaminants may be decreasing, and that […]